Organic electroluminescence (EL) display apparatus have recently attracted a great deal of attention. Pixels in the display apparatus contain light-emitting devices, i.e., organic EL display elements with thin-film solid-state stacked structures but the display apparatus are not required to have light sources such as rear or front light sources in contrast with liquid crystal display apparatus necessarily provided with them. Thus, the display apparatus are lighter in weight, thinner in thickness and stronger against mechanical shocks than the liquid crystal display apparatus.
The EL display elements are driven by driver transistors of pixel circuits, the gate electrodes of which are supplied with video signals. In other words, electric currents corresponding to the video signals are provided for the EL display elements to emit light with brightness in response to such electric currents.
Voltages at the gate electrodes of the driver transistors are provided by using a voltage or current signal supply system. The voltage-signal supply (or voltage writing) and current-signal supply (or current writing) methods are disclosed in U.S. Pat. Nos. 6,229,506B1 and 6,373,454B1, respectively.
In the pixel circuits, switching transistors are provided to supply voltages to the gate electrodes of the driving transistors. Thus, the gate voltages are controlled for a one frame period, for instance, in response to switching states of the switching transistors. Since the switching transistors turn on or off, the gate voltages are not always set to be accurate, i.e., they do not necessarily correspond to video signals precisely. One of the causes of this effect is “a feed-through (or punch-through) voltage” of the switching transistors, which is influenced by response characteristics of the switching transistors. As the gate voltages fluctuate, output current of the driver transistors also varies so that the display elements cannot always display uniform brightness in accordance with the video signal.
Meanwhile, the applicant has noticed another possible cause of the uneven brightness from the following view point. When a scanning signal is supplied to the switching transistors from the driver circuit though a scanning line, the scanning signal at an output point of the driver circuit is different in waveform from those at middle points of the scanning line (wiring) and an end terminal of the wiring, respectively. This results from the fact that time constant circuits equivalent to the respective pixel portions are different from each other. In other words, the “feed-through (or punch-through) voltage” at the output point of the driver circuit is different in amplitude from those at the middle points of the wiring and the end terminal of the wiring, respectively, to bring about uneven brightness (or uneven display) on a screen.